sergeant Bowe bergdahl is recovering at a military hospital in Germany. We learned today doctors are so worried about his mental health after five years in Taliban custody that they have told his family not to reach out even just to say welcome back. Of all the questions swirling around this young man, perhaps the most intriguing is why did he leave his base in the first place? Tonight, ABC's Martha Raddatz probes the mystery -- who is Bowe bergdahl? Reporter: This is the Bowe bergdahl the world knows. Release me, please, I am begging you. Reporter: Pleading to the camera in proof of life videos released by the Taliban. But this is the Bowe bergdahl Sherri Horton knows doing something he loved. He was a wonderful partner. All the girls enjoyed dancing with him he was strong and steady. Reporter: Sherri was bergdahl's ballet teacher and later his roommate in Idaho. He is a very interesting guy. Very quiet. He was an observer you. Couldn't put him in a cubby hole. He was kind of his own person. His own, he was very much an individual. Reporter: But tonight his family and friends can't even speak to him to say welcome back yet. The doctors at the military hospital in Germany concerned that years of isolation and captivity have resulted in deterioration of the army sergeant's mental and physical health. And he has yet to be questioned by military authorities. How did bergdahl disappear? The commander-in-chief says that is beside the point. We still get an American soldier back if he is held in captivity. Period. Full stop. Reporter: But that has not stopped the increasing questions about bergdahl's captivity. Perhaps most importantly, did the young soldier dessert. Sherri Horton doesn't know the answer to that despite her unwavering support. The Bowe I knew wasn't a quilter wasn't a deserter, this isn't what I would have seen him doing. I don't know for a fact that this is what he did. Nobody does. Reporter: But those who served right next to him in Afghanistan say he intentionally left his post. It was premeditated. It was thought out. Reporter: Cody full roomed with bergdahl before their deployment. He was not forcibly taken off the base. I don't think that somebody that deserted their platoon mates in a time of war should be haebl able to dessert and get away with it. Could he be punished? The army today said it is preparing a comprehensive review of bergdahl's disappearance and captivity, a review that will include speaking to the former prisoner. According to the chairman of the joint chiefs of staff, when bergdahl is ready to provide them we will learn the facts. Like any American he is innocent until proven guilty. Adding, our army's leaders will not look away from misconduct if it occurred. Given the years in captivity, defense officials say, punishment is unlikely. But an investigation could answer the question why would bergdahl leave the base? In the first hostage video, bergdahl was asked about soldiers who go awol or dessert. They are deserting, they know this is wrong, they just want to go home to their families. Reporter: Senior defense officials tell ABC news, bergdahl wrote a note expressing disillusionment with the movement in Afghanistan, and may have a better way to deal with it. That does not come as a surprise to nose who knew him. I guess in his mind he was dissatisfied with the united States army over there. There were times when it got to be a little much for him. He would go hike into the woods and just sit and meditate for an hour. Reporter: And controversy continues to swirl about the deal. Swapping bergdahl in exchange for five, high-level Taliban prisoners held at guantanamo, leaving many wondering did president Obama make a bad deal? Good afternoon, everybody. Reporter: Critics of the bergdahl deal say president Obama gave up too much in return for sergeant bergdahl. These Taliban operatives that we released from guantanamo were not foot soldiers. The deputy intelligence minister, two governors, a provincial chief, the top cadre of Taliban leaders. I suspect they will return to terrorism. Reporter: The men arrived in Qatar to a hero's welcome. Joined by the families. A qatari official saying the only restriction placed on them. A ban from traveling outside of Qatar for a year. Nearly a third of the detainees, left guantanamo, 200 of 600 are confirmed or suspected of returning to terrorism. Today the president defended the decision. And we have confidence that we will be in a position to go after them if in fact they are engaging in activities that threat in our defenses. Reporter: In addition, Gordon says that with the bergdahl deal president Obama may have set a dangerous precedent. If the Taliban could get five of top leaders out of guantanamo, imagine if they got a war hero. As a parent. I can't imagine the hard ship you guys have gone through. Reporter: For bergdahl's parents standing beside the president in the rose garden over the weekend. No effort seemed outside consideration. How far would a father go? His father bob grew a beard and learned the language to connect with captors, even tweeting to a Taliban spokesman just before his son's release. I am still working to free all guantanamo prisoners. ABC news has learned that a video of bergdahl which emerged earlier this year, and seen only by the U.S. Government, showed an increasingly frail and disengaged bergdahl. For Sherri Horton, it has been painful to see the dancer she once knew fade away while being held by the Taliban. He has always been a strong guy. And the last picture that were coming out. He was not. And so that's the difference I could see. At the time I actually stopped watching the videos. I want to go home. Reporter: But the army will certainly be taking a hard look again at those videos. Still so many questions remain. For "Nightline," I'm Martha Raddatz in Washington. Intriguing and mysterious

This transcript has been automatically generated and may not be 100% accurate.

Bergdahl Family Soon to Be Reunited With Their Son

Bowe Bergdahl is in a carefully managed prisoner reintegration program.

Bowe Bergdahl Prisoner Exchange Draws Fierce Criticism

The decision to release five Taliban leaders in exchange for Bergdahl breaks a longstanding policy against negotiating with terrorists.

Backlash Over Bowe Bergdahl Swap

Critics say the president did not have the authority to make prisoner exchange that brought home POW Bowe Bergdahl.

Rescue Backlash: Some Soldiers Call Bergdahl a Deserter

Some soldiers are upset about the attention surrounding Bowe Bergdahl's prison release.

Freed Taliban in Midst of Luxury in Qatar

Detainees swapped for Bowe Bergdahl get hero's welcome.

Controversy Arises About US POW Release

The president faces criticism after Sergeant Bowe Bergdahl was released in exchange for five high-ranking Taliban officials from Guantanamo Bay.

Obama Defends Deal To Release Bowe Bergdahl

President Obama is facing criticism for releasing five Guantanamo Bay detainees.

Bergdahl Too Fragile for Desertion Questions, Official Says

Official: Army Sgt. left a note before walking away from Afghan post.

Vets Accuse Bowe Bergdahl of Walking Away

"We were all pissed off," SF member says of deadly search for Bergdahl.

The Confusing Case of Bowe Bergdahl

Should we be freeing the terrorists Bergdahl presumably risked his life to kill?

Filmmaker Describes Being Held by Same Taliban Group as Bergdahl

Sean Langan was held for almost four months in 2008.

Obama Cites 'Sacred Rule' in Defense of Bergdahl Release

"We don't leave our men or women in uniform behind," he says.

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